error type of tokenizer in __init__ definition (#10879)

the orignal code in line 246 is
```
tokenizer: Optional["PreTrainedTokenizerBase"] = None,
```

it should be
```
tokenizer: Optional[PreTrainedTokenizerBase] = None,
```

src/transformers/trainer.py

@@ -243,7 +243,7 @@ class Trainer:
         data_collator: Optional[DataCollator] = None,
         train_dataset: Optional[Dataset] = None,
         eval_dataset: Optional[Dataset] = None,
-        tokenizer: Optional["PreTrainedTokenizerBase"] = None,
+        tokenizer: Optional[PreTrainedTokenizerBase] = None,
         model_init: Callable[[], PreTrainedModel] = None,
         compute_metrics: Optional[Callable[[EvalPrediction], Dict]] = None,
         callbacks: Optional[List[TrainerCallback]] = None,